1. Field of the Invention
The present invention relates to a liquid crystal display device, and more particularly, to an alignment layer for initial alignment of liquid crystal in a liquid crystal display device.
2. Discussion of the Related Art
Ultra-thin flat panel displays include a display screens having a thickness of not more than a few centimeters. Of the available types of ultra-thin flat displays, liquid crystal displays are used in a wide range of applications, such as notebook computers, monitors, spacecraft and aircraft. Liquid crystal displays have low power consumption due to their low operating voltages and are light in weight making them easily portable.
A liquid crystal display of the related art includes a color filter substrate having a color filter layer formed thereon, a thin film transistor substrate arranged opposite to the color filter substrate and having thin film transistors formed thereon, and a liquid crystal layer interposed between the two substrates.
The alignment direction of the liquid crystal layer in the liquid crystal display may be varied in response to an applied voltage, allowing light transmittance through the liquid crystal display to be controlled to display images on a screen. Electrodes are formed on the thin film transistor substrate and the color filter substrate for applying voltages to display images. Specifically, in the case of a twisted nematic (TN) mode, pixel electrodes are arranged on the thin film transistor substrate, and a common electrode is arranged on the color filter substrate, allowing a vertical electric field to be generated between the pixel electrodes and the common electrode. In the case of an in-plane switching (IPS) mode, the pixel electrodes and common electrodes are arranged in parallel on the thin film transistor substrate, thereby allowing a horizontal electric field to be generated between the pixel electrodes and the common electrodes.
FIG. 1 is an exploded perspective view of a TN mode liquid crystal display of the related art.
As shown in FIG. 1, gate lines 12 and data lines 14 crossing each other are formed on a thin film transistor substrate 10. Thin film transistors (T) are formed at crossings of the gate and data lines, and pixel electrodes 16 are connected to the thin film transistors (T). In addition, a light-blocking layer 22 is formed on a color filter substrate 20 to prevent light leakage, an RGB color filter layer 24 is formed on the light-blocking layer 22. A common electrode 25 is formed on the color filter substrate 20. In an IPS mode display, common electrodes and pixel electrodes are formed on the same substrate.
A vertical electric field generated between the pixel electrodes 16 formed on the thin film transistor substrate 10 and the common electrode 25 formed on the color filter substrate 20 can control the alignment direction of a liquid crystal layer formed between the substrates 10 and 20.
The substrates 10 and 20 are attached to each other to form a liquid crystal panel. A liquid crystal layer is formed between the substrates 10 and 20.
In a liquid crystal layer that is not aligned, the liquid crystal molecules of the liquid crystal layer are randomly arranged between the substrates 10 and 20 and have random orientations. Although not shown in the figure, an alignment layer is formed between the thin film transistor substrate 10 and the color filter substrate 20 to provide an initial alignment of the liquid crystal layer.
A related art process for forming the alignment layer for initial alignment of a liquid crystal layer includes rubbing alignment.
Rubbing alignment includes the steps of applying an organic polymer, such as polyimide, to a substrate to form a thin film, curing the thin film, and rolling a rubbing roll covered with a rubbing cloth on the thin film organic polymer. The rubbing alignment arranges the chains of the organic polymer in a particular direction determined by a movement direction of the rubbing roll.
A liquid crystal layer is aligned in the same direction in which the chains of the organic polymer are arranged by the rubbing alignment. That is, the movement direction of the rubbing roll is the same as the alignment direction of the liquid crystal.
However, the related art rubbing alignment has the following disadvantages.
First, when the arrangement of the rubbing cloth is non-uniform, regions allowing light leakage may be created in the liquid crystal layer during the rubbing alignment. FIG. 2 is a perspective view schematically illustrating a rubbing cloth having a non-uniform arrangement.
As described above, elements such as thin film transistors, color filter layers and electrode layers may be formed on a substrate. As illustrated in FIG. 2, when a rubbing roll 30 is rolled over elements formed on a substrate 10 or 20, parts 32a of a rubbing cloth 32 covering the rubbing roll 30 may be disturbed into a non-uniform arrangement. The non-uniform arrangement of the rubbing cloth 32 causes a non-uniform array of chains of an organic polymer in regions of the substrate rubbed by the disturbed parts 32a of the rubbing cloth. As a result, the alignment of the liquid crystal layer is not uniform, resulting in light leakage during operation of the liquid crystal display.
Secondly, in regions where a rubbing cloth does not contact the substrate, light leakage may occur. FIG. 3 is a perspective view schematically illustrating an arrangement in which a rubbing cloth does not contact a region of the substrate.
As described above, the elements formed on a substrate may include electrode layers, such as pixel and common electrodes. As illustrated in FIG. 3, the vertical edge of an electrode layer formed on a substrate 10 creates a region (region “A”) in which a rubbing cloth 32 cannot contact the substrate 10 during a rubbing alignment. As a result, the alignment of a liquid crystal layer is not uniform in region “A”, resulting in light leakage during operation of the liquid crystal display.
The rubbing alignment method of the related art may create regions where the liquid crystal layer is not properly aligned in regions where the alignment layer is rubbed using a rubbing cloth having a non-uniform arrangement or in regions where the rubbing cloth does not contact the substrate. These regions of improperly aligned liquid crystal cause light leakage problems during operation of the liquid crystal display.